Traditional charged particle accelerators, such as cyclotrons, which depend upon magnetic fields for acceleration and focusing of the charged particle beam are massive and expensive, limiting their application to research laboratories. Further, the available beam from such a magnetically controlled device can not be focussed narrowly enough for many applications.
In the 1970's, two Russian scientists introduced a dramatically new concept for accelerating charged particles. Instead of relying on magnetic fields, charged particles were accelerated in a linear accelerator (linac) by subjecting them to high frequency alternating electric fields, established using four poles (a quadrupole). This device is known as a radio-frequency quadrupole (RFQ) accelerator or RFQ linac. As developed and improved over the years, RFQ accelerators have been used to accelerate ions and other charged particles from energies of a few tens of kilo electron volts (keV) per atomic mass unit (AMU) up to energies of a few million electron volts (MeV) per AMU. Compared to previous accelerators, RFQ accelerators provide for relatively simple construction and operation, compactness, lightweight, and portability. RFQ accelerators will accept large quantities of ions with low kinetic energies and accelerate them to much higher energies.
Modern RFQ linear accelerators typically consist of a radio-frequency resonator with four-pole symmetry about a centerline axis and are divided into two basic classes: a 4-vane geometry and a 4-rod geometry.
The 4-vane RFQ consists of a cylindrical, square, or rectangular box divided longitudinally into four quadrants by partitions called vanes. The vanes originate at an inner wall of the box and protrude toward the centerline axis. Each quadrant of the RFQ is a separate rf resonator and the combination of the four is used to provide an rf electric quadrupole field within a cylindrical region near the axis of the structure which forms the ion beam channel. The rf field both focuses and accelerates the ion beam. In particular, in a 4-vane RFQ linac, current at radio frequencies is applied to current loops that transversely protrude into each quadrant of the linac. The currents create alternating magnetic fields within the linac, with the flux lines substantially parallel to the longitudinal axis of the vanes. A space between the base of each vane and an inner wall of the housing where the vane is fastened to the wall allows for the coupling of the magnetic fields between adjacent quadrants. The alternating magnetic fields, in turn, induce quadrant currents that alternately charge and discharge the tips of the vanes. The alternating charge on the vane tips provides means for accelerating a charged particle along the ion beam channel.
Since the 4-vane RFQ is an rf cavity resonator, the wavelength of the desired resonant frequency determines the physical dimensions of the device. This constrains the size and resonant frequencies for which the 4-vane RFQ can be designed. Because the physical dimensions of the 4-vane RFQ determines its rf resonant frequency, 4-vane RFQ's can have only a single, fixed resonant frequency. This limits its flexibility. Further, in order to obtain a proper "tuning" of a 4-vane RFQ linac, that is to say an alignment of the vanes and other parts so as to achieve the desired rf resonant frequency and mode and a high quality beam of charged particles, the transverse position of the vanes must be accurate to within about 7 parts per million. This accuracy is relatively easy to achieve at low resonant frequencies. However, at higher frequencies, and therefore smaller wavelengths and smaller RFQ dimensions, manufacturing and measurement tolerances limit the practical upper frequency attainable. The practical upper frequency of a 4-vane RFQ is about 500 MHz. Higher frequency RFQ's are presently beyond the state-of-the art of fabrication and mechanical alignment techniques.
The 4-rod RFQ consists of four rods or bars supported by transverse structures that form an inductance of a resonant circuit. A corresponding resonant capacitance comes from rod-to-rod electric fields. In general, the 4-rod RFQ is less efficient than the 4-vane RFQ. However, because the inductance and capacitance functions are separated by the resonator structure, the physical dimensions of the 4-rod RFQ need not be related to the wavelength of the resonant frequency. Thus, the physical dimensions are not determined by the wavelength of the resonant frequency as in the 4-vane RFQ and RFQ's with operating frequencies higher than 500 MHz can be fabricated as 4-rod devices. In practice, however, the longitudinal separation of the inductive rod supports cannot exceed about 14% of a wavelength without inducing unacceptable longitudinal field irregularities. Hence, the number of rod supports must increase with frequency (as the wavelength decreases), which means that the inductance of each support must be decreased in order to keep the inductance of the combination constant and thereby maintain the proper resonant frequency. The physical dimensions of the inductor supports must therefore decrease with increasing frequency. This size reduction leads to increased rf power loss and increased transverse field instability due to parasitic electromagnetic fields. Hence, low frequency RFQ's (below 150 MHz) tend to be 4-rod devices while high frequency RFQ's (above 300 MHz) tend to be 4-vane devices.
In any RFQ, the three lowest order resonant modes (the zero-order modes) consist of one quadrupole and two dipole modes. In a 4-vane RFQ, the zero-order dipole modes are lower in frequency than the zero-order quadrupole mode. In a 4-rod RFQ, the quadrupole mode is lowest in frequency. The desired operating mode for efficiently focusing and accelerating charged particles along the length of the RFQ is the zero-order quadrupole mode that has a uniform rf field intensity along the length of its structure. In practice, however, the rf fields within an RFQ are a mixture of the fields due to various quadrupole and dipole modes. Thus, within current RFQ's, the dipole component should be less than 5% of the quadrupole component and the rf fields should be constant to within 10% in the longitudinal direction. For high-quality charged particle beams, the dipole component should be less than 2% of the quadrupole component and the rf fields should be constant to within 2% in the longitudinal direction. Delicate tuning is necessary to achieve this result.
Further, in current RFQ's, coupling of dipole and quadrupole modes within an RFQ is primarily responsible for both longitudinal and transverse field errors. The potential for coupling between the desired quadrupole and undesired dipole modes is proportional to the frequency separation of these modes. The greater the separation, the less the potential for coupling and the greater the potential for higher quality tunable beams. Coupling between longitudinal dipole and quadrupole modes can be a particularly severe problem in "long" RFQ's since the frequency separation between longitudinal modes is inversely proportional to the length of the RFQ. The longer the RFQ, the greater the probability of coupling by higher-order dipole modes even at the frequency of the lowest-order quadrupole mode.
A need therefore exists for an RFQ design which improves efficiency, increases the resonant frequency range, and facilitates tuning all the while preserving the ruggedness, compactness, focusing, and simplicity features of prior RFQ designs. The present invention satisfies these needs.